1. Field of the Invention
The present invention relates to a method and system for interconnecting two three-phase alternating electric networks operating synchronously or asynchronously at alternating voltages of equal or adjacent frequencies. To carry out such interconnection, the method and system use interconnections in quadrature.
In the present specification and in the appended claims, the term "network" is used to designate both independent three-phase electric networks for the transport or distribution of electric energy or portions of such networks.
2. Brief Description of the Prior Art
Most of the systems for interconnecting two alternating electric three-phase networks presently in operation are constituted by direct current interconnection ties comprising electric power converters carrying out from one network to the other an AC-DC-AC conversion of the electric current. Such an interconnection system has the disadvantage to require compensators for neutralizing the internal reactive power in order to maintain the voltage of the interconnection points at a desired level, as well as filters for attenuating the harmonics generated by the interconnection system.
In the U.S. Pat. No. 4,621,198 (ROBERGE et AL) issued on Nov. 4th, 1986, to the same Applicant, interconnection of two three-phase networks (each comprising phases A, B and C) by means of a plurality of variable reactive impedances is described. The variable impedances are operated to establish (a) three first interconnections, each having a first susceptance, between the phases A, the phases B and the phases C of the first and second networks, respectively, (b) three second interconnections, each having a second susceptance, between the phase A of the first network and the phase C of the second network, between the phase B of the first network and the phase A of the second network, and between the phase C of the first network and the phase B of the second network, respectively, and (c) three third interconnections, each having a third susceptance, between the phase A of the first network and the phase B of the second network, between the phase B of the first network and the phase C of the second network, and between the phase C of the first network and the phase A of the second network, respectively. A regulator measures electric parameters associated with at least one corresponding phase of the first and second networks and operates the three-phase variable reactive impedances in accordance with the so measured parameters in order to vary the susceptances of the interconnections. By appropriately varying the first, second and third susceptances, a desired transfer of active power from one of the two networks to the other can be established, while maintaining the reactive power engaged in such a transfer of active power at a desired level.
A drawback of the interconnection system of U.S. Pat. No. 4,621,198 is that the ratings of the elements of the interconnection system, in particular the variable impedances, must be selected to support a relatively important quantity of reactive power (vars) for each watt of active power transferred. The rating vars/watt of the impedances is accordingly high, thereby increasing the costs of the interconnection system.